Concepts from ideal observer theory will be used to study auditory category learning and speech perception. Ideal observers provide a benchmark for assessing human performance on perceptual tasks and may serve as an appropriate starting point for developing models of actual performance. In the first set of experiments, listeners will learn to categorize novel non-speech stimuli. Prior probabilities of the categories and degree of category overlap will be varied to assess listeners' sensitivity to the distributional properties of the stimuli. The second set of experiments will test whether selective adaptation and contrast effects in speech and non- speech perception can be modeled as changes in the listeners' prior probability and stimulus likelihood distributions. The third set of experiments will extend recent pilot work in our laboratory aimed at developing ideal classifiers for naturally produced speech sounds. The results of this pilot work suggest that listeners adopt a strategy of optimizing categorization performance with respect to the natural distributional properties of phonemes in their language. This hypothesis will be comprehensively tested in these experiments. The fourth set of experiments will examine how observers weight and integrate multiple sources of evidence in making phoneme category judgments. The aim is to learn how closely listeners approximate optimal informational weightings in such tasks when different stimulus properties are degraded in either predictable or unpredictable ways. In the fifth set of experiments, natural phoneme distributions will be used to estimate optimal trajectories between a set of native language (L1) phoneme contrasts and a competing set of contrasts in a second language (L2). Perception data of L2 learners at varying stages of proficiency will be compared to the estimated optimal trajectories. The techniques developed and tested in the proposed studies can serve as important tools for the study of speech perception in listeners with varying degrees of hearing loss, including those with cochlear implants. In particular, the derivation of ideal classifiers whose auditory input is restricted to mimic that of particular hearing-impaired individuals may allow investigators to distinguish between limits on performance due to sensory factors and those attributable to non-optimal categorization strategies. [unreadable] [unreadable] [unreadable]